1. Field of the Invention
This invention relates to a method of recording a radiation image on a stimulable phosphor sheet, exposing the stimulable phosphor sheet to stimulating rays which cause it to emit light in proportion to the stored radiation energy, photoelectrically detecting the emitted light to obtain an electric image signal, and reproducing a visible radiation image from the electric image signal. This invention more particularly relates to a radiation image recording and reproducing method in which the stimulable phosphor sheets are circulated and reused for recording radiation images.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to a radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store a part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted from the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed for example in the applicant's Japanese Unexamined Patent Publication No. 56(1981)-11395 and U.S. Pat. No. 4,258,264, it has been proposed to use a stimulable phosphor in a radiation image recording and reproducing system. Specifically, a sheet comprising a layer of the stimulable phosphor is first exposed to a radiation passing through an object to have a radiation image stored thereon, and is then scanned with stimulating rays such as a laser beam which cause it to emit light in the pattern of the stored image. The light emitted by the stimulable phosphor sheet upon stimulation thereof is photoelectrically detected and converted to an electric image signal, which is processed as desired to reproduce a visible image on a recording medium such as a photographic film or on a display device such as a cathode ray tube (CRT).
The radiation image recording and reproducing system using a stimulable phosphor sheet is advantageous over conventional radiography using a silver halide photographic material in that the image can be recorded over a very wide range (latitude) of radiation exposure. More specifically, since the amount of light emitted upon stimulation after the radiation energy is stored on the stimulable phosphor varies over a very wide range in proportion to the amount of energy stored thereon, it is possible to obtain an image having desirable density regardless of the amount of exposure of the stimulable phosphor to the radiation, by reading out the emitted light with an appropriate gain, converting it to an electric image signal and using the electric image signal to reproduce a visible image on a recording medium such as a photographic light-sensitive material or on a display device such as a CRT.
The system is also advantageous in that after the radiation image information stored on the stimulable phosphor sheet has been read out and converted into an electric image signal, the electric image signal can then be easily processed in the manner most appropriate for obtaining a radiation image suitable for viewing, particularly for diagnostic purposes, when it is used for reproducing a visible image on a photographic light-sensitive material or on a display device such as a CRT.
In this radiation image recording and reproducing system, the stimulable phosphor sheet is used only for temporarily storing the radiation image in order to reproduce the final visible image therefrom in a final recording medium. For economical reasons, therefore, it is desirable that the stimulable phosphor sheets be used repeatedly.
In order to reuse stimulable phoshphor sheets as described above, the radiation energy remaining on the stimulable phophor sheet after it is scanned with stimulating rays to read out the radiation image stored thereon should be eliminated or erased as described, for example, in Japanese Unexamined Patent Publication No. 56(1981)-12599 and U.S. Pat. No. 4,470,619. The stimulable phosphor sheet can then be used again for radiation image recording.
The applicant therefore proposed in Japanese Patent Application No. 58(1983)-66730 a radiation image recording and read-out apparatus comprising:
(i) a circulation and coveyance means for conveying at least one stimulable phosphor sheet for recording a radiation image thereon along a predetermined circulation path,
(ii) an image recording section positioned on said circulation path for recording a radiation transmission image of an object on said stimulable phosphor sheet by exposing said stimulable phosphor sheet to a radiation passing through said object,
(iii) an image read-out section positioned on said circulation path and consisting of a stimulating ray source for emitting stimulating rays for scanning said stimulable phosphor sheet carrying said radiation image stored thereon in said image recording section, and a photoelectric readout means for detecting light emitted from said stimulable phosphor sheet scanned with said stimulating rays to obtain an electric image signal, and
(iv) an erasing section for, prior to the next image recording on said stimulable phosphor sheet for which the image read-out has been conducted in said image read-out section, having said stimulable phosphor sheet release the radiation energy remaining on said stimulable phosphor sheet;
whereby said stimulable phosphor sheet is circulated through said image recording section, said image read-out section and said erasing section and reused for radiation image recording.
When the stimulable phosphor sheets are reused in this way, however, they suffer gradual deterioration of their physical and chemical properties. Since it is not possible to obtain a high quality reproduced image from a sheet that has been excessively degraded through repeated use, it is necessary to replace the worn-out sheets with new ones. In a system in which the stimulable phosphor sheets are reused by circulating them through an image recording section, a image read-out section and an erasing section, however, it is generally impossible to identify which of the stimulable phosphor sheets circulating through the system have deteriorated to the point that they require replacement. This is because the cover of the radiation image read-out section has to be kept closed at all times during use so that when, as is almost always the case, a plurality of sheets are maintained in circulation within the system in order to enhance the efficiency of radiation image recording and read-out, it is virtually impossible to identify which sheet a particular reproduced image was obtained from.
This problem arises not only when the stimulable phosphor sheets are reused by circulating them through an integrated system such as described in the foregoing but also when image recording, read-out and reproduction are carried out on a plurality of stimulable phosphor sheets at separate apparatuses as when the work of processing the sheets is divided among a number of operators. In such cases, it becomes particularly difficult to ascertain the degree of degradation of the individual sheets when the electric image signals obtained by image read-out are once stored on a recording medium such as a magnetic disk and the visible images are later produced from the stored image information.